1. Field of the Invention
The invention relates generally to the field of marine seismic surveying. More particularly, the invention relates to structures for a marine seismic energy source and an array of seismic energy sources.
2. Background Art
In marine seismic surveying, a seismic energy source is used to generate seismic energy in the form of acoustic pulses or waves in a body of water such a lake or the ocean. The seismic energy travels downwardly in the water, through the water bottom, and through the Earth formations underlying the water bottom. Part of the energy passing through the Earth formations underlying the water bottom is reflected upward from acoustic impedance boundaries in the Earth formations. The upwardly traveling seismic energy is detected by sensors such as hydrophones towed in one or more streamer cables disposed near the water surface, or by sensors disposed in cables along the water bottom. The sensors convert the detected energy to electrical or optical signals. The electrical or optical signals are then conditioned and interpreted to provide information both as to the composition and the structure of the various subsurface Earth formations. Such information is used particularly to determine the possibility that such Earth formations may contain mineral deposits such as hydrocarbons.
The most frequently used marine seismic energy source at present is known as an “air gun.” In an air gun, a charge of compressed air or inert gas under high pressure, on the order of 2000-6000 pounds per square inch (130 to 400 bar), is supplied to a chamber within the gun. When the air gun is actuated or “fired”, a valve rapidly opens to discharge the pressurized air or gas from the chamber into the water. The valve then closes and the chamber is repressurized with air or gas from a source such as a compressor. Such firing can be repeated as often as desired and within the capacity of the source to repressurize the chamber. The air or gas discharges in a bubble or plurality of such bubbles in the water.
A single air gun produces a seismic pulse having acoustic energy content with respect to frequency (the source “signature”) related to a complex pressure interaction between the air bubbles and the water that causes the bubbles to oscillate as they float toward the water surface. Such interaction can produce extraneous bursts of seismic energy following the initial energy burst. The amplitude and periodicity of these bubble-generated extraneous bursts depend on, among other factors, the depth of the gun in the water and the size of the pressurized air chamber in the gun. It is therefore known in the art to use an array of air guns having various different chamber sizes, and to fire such guns contemporaneously or in a preselected timing sequence. Such firing of an array of air guns provides several advantages over firing a single air gun. First, the total amount of energy being imparted into the Earth's subsurface for each seismic “shot” is increased. In addition, the different chamber sizes for the various guns will produce different bubble responses, causing the bubble responses to tend to cancel each other. The directivity of the energy source toward the water bottom can be improved, because in directions other than directly below the source array, some frequencies in the seismic energy will be attenuated by the spatial distribution of the guns in the array. Design considerations for enabling air gun arrays to attain certain spectral and directivity characteristics are disclosed in U.S. Pat. No. 4,064,479 issued to Ruehle, and in U.S. Pat. No. 4,382,486 issued to Ruehle, for example.
A typical air gun array is towed behind a seismic survey vessel or a source vessel using a structure suspended from a buoy or similar flotation device that can be towed in the water. The flotation device is typically coupled to a frame or similar substantially rigid structure so as to suspend the frame in the water. Individual air guns forming the array may be suspended from the frame by cables or chains. An example of a typical air gun array known in the art is shown in FIG. 1. A tow cable 10 is functionally coupled at one end to the towing vessel (not shown). Near the aft end of the tow cable 10, the tow cable 10 may be functionally coupled to a float 12. The float 12 is coupled to frame struts 14 that form a rigid framework for the tow cable 10. The tow cable 10 is shown in FIG. 1 as being substantially U-shaped at the aft end and held in such configuration by the struts 14. An air gun mounting frame 17 may be coupled to the tow cable 10 proximate a lower end of the struts 14. Air guns 22 are suspended from the mounting frame 17 by chains 24 or a similar fixed length device. Typically, the mounting frame 17 will include one or more depth sensors 18 and, proximate the positions where the air guns 22 are suspended, a hydrophone 20 or similar seismic sensor to measure the near field acoustic signature of each of the air guns.
In air gun arrays known in the art, individual air guns in the array are typically suspended at a substantially fixed depth in the water, which depends on the length of the struts 14 and the chains 24. However, it is desirable in some instances to be able to change the depth of one or more of the air guns during actual survey operations in order to fine tune the spectral content of the array, among other purposes. When using seismic source towing structures known in the art it is necessary to remove the towing structure from the water and change the length of a rope, cable, chain or other device that couples the source to the towing frame or structure. Such operation is time consuming, and does not provide for adjustment of the source depth during surveying operations. What is needed is a device to enable adjusting the depth of a seismic energy source during survey operations.
In some cases it may also be desirable to be able to move the lateral position of the seismic sources with respect to the towing vessel, among other purposes, to provide different illuminating angles of the seismic energy with respect to the subsurface Earth formations. Using source array structures known in the art, it is not possible to move the source lateral position during operation of the seismic source array.